Ultra-low thermal expansion materials such as ULE® glass (a trademark of Corning Incorporated) and Zerodur® glass (a trademark of Schott Glas) provide dimensional stability for a variety of precision applications including structures requiring significant dimensional stability over a range of temperatures. Examples include structural materials for precision machines and instruments and substrate materials for space optics, telescopes, and extreme ultra-violet lithography (EUVL) optics and photomasks.
For such purposes as calibration, certification, and process feedback, precise measurements are required of the thermal expansion characteristics of these ultra-low expansion materials (referred to as coefficient of thermal expansion measurements or CTE measurements). Since few if any materials exhibit lower thermal expansion characteristics, the instruments used for measuring ultra-low thermal expansion materials are often subject to nearly the same or even greater thermally induced dimensional variations. The CTE measurements are taken at different temperatures by heating or cooling the test material within the measuring instrument. Accompanying thermal deformations of the measuring instrument are generally the largest source of uncertainty in the CTE measurements.
Prior CTE measuring instruments that measure dimensional changes in ultra-low thermal expansion materials using the mechanism of interference have attempted to compensate for such instrument errors in two main ways. Some employ common path interferometry so that local instrument changes equally affect the common path portions of test and reference beams. However, these interferometers have difficulty consistently relating the test and reference beams where the beams depart in the vicinity of the test materials under investigation. Joints and other connections between reference surfaces and the test materials are also significant sources of error. Other CTE measuring instruments include two separate interferometers, one for measuring the test material and the other for simultaneously measuring both the test material and the instrument. The simultaneous measurements are taken along parallel paths. However, the instrument measuring path does not account for all of the spurious variation undergone by the test material measuring path.